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Leaked reports by NASA engineers query the U.S. space agency's decision to wind up the Hubble Space Telescope.

The two reports maintain it is no riskier to service the orbiting telescope than to use shuttle astronauts to finish building the International Space Station, which will require some 25 shuttle flights.

The reports, provided to Reuters by an astronomer familiar with the case, dispute NASA's January decision to forgo a scheduled shuttle mission to repair and upgrade Hubble in 2005 or 2006.

"The final planned Hubble Space Telescope servicing mission, SM4, will be at least as safe as shuttle flights to the International Space Station," one of the reports said.

The other report argued that missions to Hubble would have the same ability as those to the station to deal with possible damage to the shuttle's thermal protection system.

Damage to this system on lift-off doomed shuttle Columbia to break up on re-entry in February 2003, killing all seven astronauts.

Both reports were written by NASA engineers who feared they would lose their jobs if their names were made public, said the astronomer, also requesting anonymity.

Without the servicing mission, the gyroscopes that enable Hubble to point at specific objects will eventually fail and the spacecraft's batteries will fade. When that happens, Hubble will be nudged out of orbit and down toward Earth, where it will burn up on re-entry.

The servicing mission, which would also install a new camera and another instrument, could give Hubble six more years of effective life, said Steve Beckwith, director of the Space Telescope Science Institute, which manages Hubble.

Will NASA reconsider?

"I hope very much that NASA will reconsider this decision in light of these new analyses that have come out in the press," Beckwith said, after a report of the engineers' analyses was published in The New York Times.

Calls to NASA for comment were not immediately returned.

NASA Administrator Sean O'Keefe said he decided to scrap the service mission based on recommendations from the Columbia Accident Investigation Board, which investigated Columbia's breakup.

The board said the shuttles should be able to inspect and repair problems before re-entering Earth's atmosphere, and could take advantage of additional resources when they are near to or docked with the space station.

O'Keefe has said missions to Hubble would not provide the same kind of "safe haven" for astronauts in case of emergency that the space station would, and developing technologies to make trips to Hubble safer for a single servicing flight would be impractical.

In response to pressure from Hubble supporters, O'Keefe agreed to ask for a second opinion. But he has said repeatedly that this is a judgment call that ultimately is his to make.

"I think basically Mr O'Keefe has overestimated the risk and underestimated the benefits of flying a servicing mission to Hubble," Beckwith said.